For quick reference, I'm going to copy a bit of relevant content from the FAQ on the bottom of the page...

As of 6.3, the X-Pack code will be opened under an Elastic EULA. However, it will not be ‘Open Source’ as it will not be covered by an OSI approved license. The interaction model for open X-Pack will be identical to the open source Elastic Stack, including the ability to inspect code, create issues and open pull requests via our existing GitHub repositories. The specific language for the license is still being finalized and will be added to the repositories in the near future.